Letters to the Editor

ing from a trisomic zygote would not seem likely, since it might be expected that the phenotype of the affected individual would be likely to contain many of the features of DS. In light of the potential importance of the above issues, there would seem to be a good case for conducting further studies of maternal age effects in AD and mosaic DS. ROGER HOWELLS Section of Neuropsychiatry Institute of Psychiatry London References De Braekeleer M, Froda S, Tetreault H, Gauvreau D (1988) Parental age and birth order in Alzheimer's disease: a case-control study in the Saguenay-Lac-St-Jean area (Quebec, Canada). Can J Neurol Sci 15:139-141 Potter H (1991) Alzheimer disease and Down syndromechromosome 21 nondisjunction may underlie both disorders. Am J Hum Gen 48:1192-1200 Sachs ES (1971) Trisomy G/normal mosaicism, a cytological and clinical investigation. Stenfert Kroese, Leiden o 1992 by The American Society of Human Genetics. All rights reserved. 0002-9297/92/5006-0027$02.00

Am. J. Hum. Genet. 50:1343, 1992

Reply to Howells To the Editor:

Dr. Howells, in his letter to the editor of The American Journal of Human Genetics, raises the very interesting point that, if Alzheimer disease originates from somatic cell mosaicism for trisomy 21, then a maternal age effect similar to that seen for Down syndrome should not occur. The possible influence of parental age on the development of Alzheimer disease has been most recently investigated by Farrer et al. (1991). In this careful study, a total of 237 patients with Alzheimer disease were each matched with five control subjects on the basis of sex, year of birth, survival age, and location of residence. No significant effect of advanced maternal age on the risk for Alzheimer disease was found. However, decreased paternal age increased somewhat the susceptibility to late-onset Alzheimer disease. Dr. Farrer and his colleagues suggest that the results might be explained by genetic im-

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printing, but they point out that social or environmental influences are hard to rule out. HUNTINGTON POTTER AND LISA N. GELLER Department of Neurobiology

Harvard Medical School Boston Reference Farrer LA, Cupples LA, Connor L, Wolf PA, Growden JH (1991) Association of decreased paternal age and lateonset Alzheimer's disease: an example of genetic imprinting? Arch Neurol 48:599-604 i 1992 by The American Society of Human Genetics. All rights reserved. 0002-9297/92/5006-0028$02.00

Am. J. Hum. Genet. 50:1343-1348, 1992

Ataxia-Telangiectasia: Linkage Evidence for Genetic Heterogeneity To the Editor:

The recent paper by Foroud et al. (1991) investigates genetic linkage between the recessive disease ataxiatelangiectasia (AT) and a number of chromosome 11 q markers. While the evidence for linkage to 1 1q23 markers displayed by the 111 families in this international study is overwhelming, a cautious interpretation as to the exact location of the AT gene or genes is warranted because of the existence of complementation groups in AT. Three complementation groupsA, C, and D-make up 55 %, 28%, and 14%, respectively, of tested patients (Jaspers et al. 1988). Other, less common complementation groups account for the remaining 3 % of affecteds. Sufficiently many group A and group C families are known that group A and group C genes can be confidently localized to the 11q23 region (Gatti et al. 1988; Ziv et al. 1991). Furthermore, the radiosensitivity of a group D fibroblast line has been corrected by introducing a normal chromosome 11 (Komatsu et al. 1990) and an 11q fragment (Ejima et al. 1990) into these cells. Recently, Kapp et al. (1991, 1992), who have reported cloning of a putative group D gene that maps, by means of radiation hybrids, to the region between markers THY1 and D11S83. This position, although still within the 11q23 region, is more distal than the

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Ataxia-telangiectasia: linkage evidence for genetic heterogeneity.

Letters to the Editor ing from a trisomic zygote would not seem likely, since it might be expected that the phenotype of the affected individual woul...
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